Electron multiplier



Feb, 11, 1941. R. L. SNYDER JR ELECTRON MULTIPLIER Filed April 30, 1938 Juventor 611,9 den Jr.

Richard L Gttorneg Patented Feb. 11, 1941 ELECTRON IVIULTIPLIER Richard L. Snyder, Jr., Glassboro, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1938, Serial No.205,208

Claims.

This invention relates toelectron discharge devices, particularly to electron multipliers, and has special reference to the provision of improvements in the last multiplying and output 5 stages of multi-stage electron multipliers.

One problem encountered in the design of multi-stage electron multipliers is in the design of a collecting electrode capable of handling efficiently large output currents. Perhaps the most efficient electron collecting multiple electrode known heretofore is the modified L- shape anode described by Rajchman and Pike in copending application Serial No. 171,916, filed October 30, 1937.

The above identified application discloses an electron multiplier comprising a pair of sets of multiplying electrodes mounted on opposite sides of an axis which extends between the cathode and anode. As described by Rajchman and Pike,

the modified L-shape contour of the anode corresponds to the contour of the equipotential surface which would be present midway, electrically, between two adjacent multiplier electrodes (in the same upper or lower set) in a device having an infinite number of multiplying electrodes.

As set forth in the said disclosure the exact form of anode required for a'given electrode assembly may be determined by constructing a scale model in metal of the multiplying electrodes and experimentally plotting the equi-potential line of the potential applied to one of the central multiplying electrodes when the entire model is energized at the relative potentials to be applied to the corresponding electrodes of the completed electron multiplier.

Anodes of the type described have provenentirely satisfactory in electron multipliers designed to handle average output currents, but give rise to space charge limitations with consequent defocusing at relatively high current levels. Further, the relative capacitance compared to the output current derived from a given emission area is greater, in certain cases, than has been 0 found necessary or desirable.

Accordingly, a principal object of the present invention is to provide an output electrode assembly capable of handling a large power output 50 and this, too, in cases where the output voltage swing is large.

Another object of the invention is to provide an output electrode assembly wherein inter-elec.. trode capacity is as small asis consistent with 55 the intensity of the electrostatic field necessary to maintain the last secondary electron emissive electrode in a saturated condition.

The above and other objects are achieved in accordance with the invention by the provision of an output electrode assembly comprising a up cylindrical or semi-cylindrical anode mounted in relatively closely spaced, preferably parallel, relation with a curved, preferably semi-cylindrical, multiplying electrode. The generatrices of the surfaces of these terminal electrodes are par- 1 allel with each other and are normal to a plane containing the axis about which the preceding multiplying electrodes are mounted.

Certain details of construction together with other objects and advantages will be apparent, 1 and the invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein Figure l is an end plan view of a thermionic duplex electron multiplier of the type disclosed 20 in Zworykin and Snyder copending application Serial No. 187,634, filed January 29, 1938 and provided with a pair of output electrodes constructed in accordance with the principle of the present invention, and 25 Figure 2 is a partly diagrammatic longitudinal sectional View of an electron multiplier of the type disclosed in the above identified copending Rajchman and Pike application and provided with an output electrode assembly designed, po- 30 sitioned and arranged in accordance with the present invention.

As above indicated, the device of Fig. 1 is an electron multiplier of the duplex type, that is to say there are two separate sets of multiply- 35 ing electrodes and two output electrodes enclosed in a single envelope T. A conventional, indirectly-heated, cathode I, suitably mounted along the central axis of the envelope T, constitutes a copious source of primary thermionic electrons 40 for both sections A and B of the device. A control grid 2 and an accelerating grid 3 surround the cathode l and control the electrons therefrom in a known manner when supplied with proper fluctuating and direct potentials from a source, not shown. Two bent plates or bailles 4 and 5 define oppositely directed corridors through which the thermionic electrons from the cathode I enter the separate multiplier sections A and B. Ordinarily, the baffles 4 and 5 will be electrically connected to the cathode. The multiplying electrodes of section A are odd-numbered ll, l3, l5 and H. The corresponding electrodes of section B are even-numbered l2, l4,

l6 and I8, respective y. The output electrodes 5 or anodes of the separate sections are designated l9 and 20, respectively.

The separate multiplier sections are of duplicate construction and are similarly energized. Hence, the discrete streams of electrons follow congruent skip-along paths, indicated by the broken arrows, between the first and. last multiplying electrodes of each section. Accordingly, a description of one section, say section A, will serve as a description of both.

In accordance with the principle of the invention, the last multiplying electrode comprises a concave, preferably semi-cylindrical surface which is preferably provided with an outwardly bent section 8 which will be understood to assist in maintaining a desired electrostatic focusing field adjacent and between the preceding electrodes. The anode l9 comprises a convex, preferably cylindrical surface mounted with the generatrix of its curved surface substantially parallel to the generatrix of the surface of the last multiplying electrode i1 and so spaced from the emissive surface of the said multiplying electrode as to permit the secondary electrons from any point on the next-to-the-last multiplying electrode l5 to strike the cylindrical section of the last multiplying electrode without impinging upon the anode. In practice, the outside radius of the anode will ordinarily be in the neighborhood of from one-half to one-third the inside radius of the last multiplying electrode.

With the anode and last multiplying electrode designed, positioned and arranged in the manner described, the electrostatic field at the surface of the last multiplying electrode is very intense so that it is m'aintainedin a saturated condition. Because electrons from the neXt-to-the-last multiplying electrode are permitted to spread all over that part of the surface of the last multiplying electrode which faces the anode, space charge effects are avoided and it follows that all of the secondary electrons will be drawn to the anode. Because by far the greatest part of the surface area of the anode is employed as an electron collecting surface the capacitance between the anode and all of the other parts of the device is minimized, i. e., the inter-electrode capacitance is of the lowest value consistent with high electroncollecting efficiency.

While the output electrode assembly of the invention finds optimum usefulness in thermionic type electron multipliers designed to handle large output currents, the simplicity of the electrode assembly of the invention recommends its use in electron multipliers of other types. Thus, Fig. 2 shows the invention as applied to a photosensitive electrori-multiplier of a type described in the previously identified Rajchman and Pike disclosure.

In Fig. 2, 2| designates a photosensitive cathode and 22 to 30 inclusive a number of multiplying electrodes arranged in staggered relation on opposite sides of the central longitudinal axis of a highly evacuated envelope T. The electron collecting electrode or anode is designated 3|. Section aw or the cathode is of foraminous construction to permit light from an external source exemplified by the lamp L and lens L to impinge upon the photosensitive surface 2|b'. The other electrodes including the last multiplying electrode 3i! and the anode 3| are of imperforate construction.

As in the previously described embodiment of the invention, the last multiplying electrode comprises a semi-cylinder 30 and the anode 3| is in the form of a cylinder mounted with the generatrix of its curved surface substantially parallel to the generatrix of the surfaces of the last multiplying electrode and so spaced from the emissive surface of said multiplying electrode as to permit secondary-electrons from the next to the last multiplying electrode 29 to strike the cylindrical section of the last multiplying electrode 3|] without impinging upon the anode. As before, the outside radius of the anode will ordinarily be in the neighborhood of one-half to one-third of the inside radius of the last multiplying electrode.

The potential distribution required to insure optimum performance of the device in Fig. 2 may be expressed by the mathematical series IV, 2V, 3V, 4V, etc. where IV represents the potential drop between the primary electron source and the first multiplying electrode and 2V, 3V, 4V, etc. represent the potential drop between the respectively succeeding electrodes in point of elec-' tron travel and said source.

-For the purpose of providing such a potential distribution the cathode 2| may be connected to the negative terminal of a direct current source exemplified in the drawing by a resistor R, and the first multiplying electrode, 1. e. electrode 22, connected to a point |V somewhat more positive. The other electrodes 23 to 3|, inclusive, in the order of their numbers, are shown connected to successively more positive points 2V to HIV on the resistor.

The reference characters |V, 2V, 3V, 4V, etc., given to the several points on resistor R, will be understood to indicate that the voltage drop between a given electrode and the cathode is the designated whole number multiple of the drop existing between the cathode 2| and the first multiplying electrode 22. Thus, when the potential drop between the first multiplying electrode 22 and cathode 2| is volts, the drop between electrodes 23 and 2| should preferably be 200 volts, that between electrodes 24 and 2|, 300 volts.

If a beam of light, say of varying intensity, is caused to impinge upon the first lower electrode 2|, photo-electrons will be emitted in a quantity determined by the instantaneous intensity of the light beam. These photo-electrons will be accelerated toward the first upper electrode 22 and will impinge upon this first multiplying electrode. The photo-electrons striking electrode 22 will cause the emission of secondary electrons, the number of secondary electrons emitted being dependent, in part at least, upon the magnitude of the potential between it and the cathode.

The next electrode in point of electron travel is the second lower electrode 23. The trajectory of secondary electrons from the first multiplying electrode 22 is such that they impinge upon the cupped surface of the second multiplying electrode 23. Here again, a multiplication, by reason of secondary emission, is secured and this is repeated in any number of stages until the amplified stream of secondary electrons is collected upon the cylindrical output electrode 3| and caused to flow in a utilization circuit exemplified in the drawing by the impedance Z included between the output electrode 3| and the positive terminal NW of the potential divider.

While the invention has been described as embodied in thermionic and photosensitive multistage electron-multipliers of the electrostatic type, it will be obvious to those skilled in the art that the herein described output electrode assembly may be applied to magnetic type electron-multipliers without departing from the spirit and scope of the invention. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense, except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. An electron multiplier comprising a source of electrons, a concave secondary-electron emissive surface, an anode comprising a convex electroncollecting surface mounted between said source and said concave emissive surface, and electrostatic means comprising extensions of said source and emissive surface for directing the electrons from said source to said concave emissive surface in paths which avoid the said convex electroncollecting surface of said. anode.

2. An electron multiplier comprising a cathode, an anode, a plurality of secondary-electron emissive electrodes having concave emissive surfaces mounted in successive array on opposite sides of a median line which extends between said cathode and anode, said anode comprising a convex electron-collecting surface having an axis of curvature which coincides with the axis of the last of said concave emissive surfaces and which is offset from the axis of the next to the last of said concave emissive surfaces.

3. The invention as set forth in claim 2 and wherein said last concave emissive surface extends substantially 180 around said convex electron collecting surface.

4. The invention as set forth in claim 2 and wherein the outside radius of said convex electron collecting surface is substantially no less than one-third of the inside radius of said last concave emissive surface.

5. The invention as set forth in claim 2 and wherein the outside radius of said convex electron collecting surface is substantially no more than one-half of the inside radius of said last concave emissive surface.

RICHARD L. SNYDER, J R. 

